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J. Amu. (1995) 186, pp. 509-515, with 9 Jigures
Printed in GreaL Britain
509
Functional relationship between the abductor pollicis longus
and abductor pollicis brevis muscles: an EMG analysis
E. VAN O U D E N A A R D E 1 A ND R. A. B* O O S T E N D O R P 2
1 University o f Nijmegen, the Netherlands, 2Free University o f Brussels, Belgium and National Institute f o r Research and
Postgraduate Education in Physical Therapy, Amersfoort, The Netherlands
(Accepted S November 1994)
ABSTRACT
This study examined the anatomical and functional relationships between the abductor pollicis longus (APL)
and the abductor pollicis brevis (APB) muscles. The APL has 2 divisions, a distal superficial division and a
more proximal deep one. A direct anatomical connection between the extrinsic and intrinsic muscles o f the
thumb is made up by the deep division of the-APL, the APB and the trapezium. Electromyographic (EM G)
recordings were made from these muscles to investigate differences and similarities in muscle activation. The
EMG acitivity was recorded with intramuscular wire electrodes during isometric as well as dynamic
contractions in different directions, both for the thumb and for the hand. The E M G activity o f the right
hand o f 8 subjects was scaled relative to the the mean EM G value at the maximum voluntary isometric
contraction in order to compare relative muscle activity in various directions for different subjects. APB and
APL were activated in a number o f directions for the thumb as well as for the hand. Cooperation between
these muscles is necessary to stabilise the trapezium to the carpus in movements o f the thumb. APB is
activated in movements o f the hand to maintain the tension in the deep APL. The deep APL is activated in
movements of the thumb to prevent undesired movements o f the hand and the forearm. The APB and
superficial APL are prime movers o f the thumb.
Key words: Electromyography; hand function; abductor pollicis longus; abductor pollicis brevis; trapezium.
INTRODUCTION
The extrinsic muscles o f the thumb are extensor
pollicis longus, extensor pollicis brevis, flexor pollicis
longus and abductor pollicis longus. These muscles
are polyarticular and act at the carpal joint as well as
on the joints o f the thumb. The intrinsic muscles o f the
thumb are opponens pollicis, flexor pollicis brevis,
adductor pollicis and abductor pollicis brevis. These
muscles are smaller than the extrinsic muscles and
only cross the joints o f the thumb. Little is known
about the functional relationships between the ex­
trinsic and intrinsic muscles o f the thumb.
The only anatomical connection between the extrinsic and intrinsic muscles of the thumb is made up
by abductor pollicis longus (APL), abductor pollicis
brevis (APB) and the trapezium (Fig. 1). APL has
a distal superficial and a more proximal deep division
(van Oudenaarde, 1991a). The superficial division
(APLsup) inserts with one or more tendons into the 1 st
metacarpal bone (MCl). The deep division (APLdeep)
has a variable number o f insertions into the trapezium,
the joint capsule, the capsular ligaments and mostly
jnto the radial muscle belly o f APB, sometimes into
opponens pollicis (OP). APB, similar to APL, also has
more divisions (Simard & Roberge, 1988). This muscle
originates from the flexor retinaculum and the
Trapczium
Retinaculum
extensorum
Retinaculum
flexorum
Fig. 1. Anatomical connection between abductor pollicis brevis
(APB), the deep division o f abductor pollicis longus (APL) and the
trapezium.
Correspondence to D r E. van Oudenaarde, Weezenhof 62—37, 6536 AP Nijmegen, The Netherlands.
MU
E . van Oudenaarde and R. A. B . Oostendorp
trapezium and inserts into the proximal phalanx of the
thumb. Such an insertion o f one tendon into another
muscle, is rarely found in the human body (APLdeep
into APB) but is considered to be of functional
importance. A PL and APB are innervated by different
nerves, the radial and the median nerves, respectively.
The trapezium is part o f the radial chain o f the
carpus (Kauer, 1986). Kauer divided the carpus into 3
longitudinal chains; radial, central and ulnar. The
radial chain is formed by the radius, scaphoid,
trapezoid and trapezium. The joints of the radial
carpal colum n are for movements dependent on each
other as well as on the other chains o f the carpus
(Kauer, 1986). A t the distal end o f the radial column
is the trapeziometacarpal joint, otherwise referred to
as the 1st carpometacarpal joint (CMCI). This is a
highly interesting joint, being free moving in contrast
to the joints o f the radial carpal chain, APL inserts
into both sides o f CMCI, i.e. into the trapezium
(APLdeep) as well as into the M CI (APLsup).
Few facts are known as to the role o f the trapezium
in the function o f the carpus. Ruby et al. (1988)
labelled m ost o f the bones o f the carpus for kinematic
analysis, but the trapezium was not included. Kauer &
de Lange (1987) noted that the distal carpal row,
which includes the trapezoid and trapezium, can be
considered as acting as a fixed group. On the other
hand, it is known that an interosseous ligament
between the trapezium and trapezoid is usually lacking
(Warwick & W illiams, 1975; Rom anes, 1976; Spinner,
1984). Berger & Crowinshield (1982) postulated that
the trapezium is a marginal bone in the distal carpal
row and hence has less articulatory contact with the
other bones o f the distal row o f the carpus. The
trapezium would therefore have more mobility than
the other bones o f the carpus. Van Oudenaarde
(1991 b) noted displacement o f the trapezium during
passive movements o f the thumb.
The trapezium moves during dorsopalmar flexion
o f the hand in relation to the scaphoid (Kauer, 1986),
but is fixed to the scaphoid if the thumb moves. In this
way the trapezium forms the link between the
interconnected movements o f the bones o f the carpus
and the independent movements o f CM CI. The
fixation o f the trapezium to the scaphoid and the
trapezoid is a point o f some interest. The exact
mechanism o f this fixation is uncertain. From the
displacement o f the trapezium during passive movement (from observations on cadaver hands) it would
be expected that muscles play an important role in this
fixation. N o muscles insert into m ost of the bones o f
the carpus with the consequence that the movements
o f these bones in relation to each other are indirect.
An exception is the trapezium into which 2 muscles
insert, APL and APB.
The functional relationship between APL and APB,
as well as the role o f muscles in the fixation o f the
trapezium to the carpus, is the subject o f this study,
This relationship is complex and still poorly understood. As there are numerous combinations o f
movements o f the hand in the carpal joint and o f the
thumb in CMCI, the experiments in this study were
performed under controlled conditions to test the
hypothesis that both muscles and the trapezium are
functionally related. In this study EMG signals from
APL and APB were recorded in a number o f directions
to look for differences and similarities in muscle
activation in each of these muscles in the chosen
directions. The results will be used to discuss the
coordination between these muscles and the functional role o f these muscles in stabilisation o f the
trapezium to the carpus.
MATERIALS AND METHODS
The methods were used similar to those described
elsewhere (van Oudenaarde et al, 1994). A summary is
given below,
Subjects
Eight adult subjects with normal manual function
participated in the study, 4 women and 4 men, aged
from 35 to 70 y. In 1 male subject, only the EMG
signals from the hand were recorded. All experiments
were performed on the right hand,
Terminology
In this study 'isometric contraction’ is defined as
muscle activation against a constant load without
shortening or lengthening of the muscle and without
joint movement. By ‘dynamic contraction’ we refer to
contraction against a constant load in which the
thumb or the hand is free to m ove and in which the
length of the muscle can change by movements o f the
thumb or the hand.
EMG activity was recorded for isometric and
dynamic contractions in a number of directions. The
isometric contractions in each o f 6 directions and the
movements in each o f the 6 directions for the thumb
and the 7 directions o f the hand will be referred to as
‘directions1. The terminology o f the International
Federation o f Societies for Surgery o f the Hand
(Terminology for Hand Surgery, 1970) will be used to
refer to the directions o f the thumb.
Cooperation between the A P L and the A P B
Equipment
EM G signals were recorded by means o f intramuscular wire electrodes. Alternately the thumb and
the hand were connected to a strain-gauge by means o f
a thumb splint or a hand splint. The splints were
connected to a cable leading to a torque motor used to
deliver dynamic loads.
511
level of a = °-05 w as chosen. The null hypothesis was
tested that there is no tendency for the EM G signals
in one direction to be larger or smaller than in another
direction. The alternative hypothesis implies that the
outcom e o f one direction is larger or smaller than in
other directions,
RESULTS
Procedure
For the directions o f the thumb, the forearm was held
in midposition between pronation and supination, the
hand in midposition between palmar flexion and
dorsiflexion. The forearm and hand were positioned
from the elbow to the fingers along the armrest of a
chair, with the fingers slightly flexed. For dorsiflexion
and palmar (dorsopalmar) flexion o f the hand, the
thumb was held in a relaxed position on the index
finger. In the positions of pronation and supination
the right forearm rested on the armrest from the
elbow up to the wrist. In midposition the forearm
down to and including the little finger lay on the
armrest. Changes in the amount o f EMG activity in
isometric and dynamic contractions were determined
relative to the mean amplitude o f EMG activity in the
starting position by subtracting the EMG in the
starting position.
All EM G signals were recorded for a period o f 3 s.
Pronation, supination and midposition o f the forearm
were combined with dorsiflexion and palmar flexion
o f the hand in the following directions: ( 1 ) thum b :
palmar abduction, opposition, radial abduction, ad­
duction, reposition, repeated palmar abduction; (2 )
forearm and hand : supination with dorsiflexion and
palmar flexion, pronation with dorsiflexion and
palmar flexion, midposition with dorsiflexion and
palmar flexion, repeated midposition with palmar
flexion.
The mean value o f EM G signals, relative to the
maximum EM G for isometric as well as for dynamic
contractions, respectively, for the APB and the
APLdeep in various directions are presented in
Figures 2, 3, 6 and 7.
% m a x , £M G activity
Thum b
100
80
60
40
20
Fig. 2, Mean percentage of E M G activity relative to the maximum
E M G activity for isometric contractions for APB and APLdeep in
various directions. Bars indicate s . d . Hatched columns, APB; black
columns, deep division of APL, PA, palmar abduction; OP,
opposition; RA, radial abduction; AD, adduction; REP, repo­
sition; RPA, repeated palm ar abduction. * Correlation significant
(P ^ 0.05).
% m ix . EMG activity
Thumb
100 r 1
80
D ata analysis
In order to compare the function o f APL and APB,
the percentages o f EMG activity relative to the
maximum EMG activity were calculated for each
direction and for each muscle (separately for the
directions of the thumb and those of the hand).
Finally the results from all subjects were averaged and
the s .d . was calculated.
Differences between the amount o f EMG activity in
the various directions were tested with the Wilcoxon
Matched Pairs Test (Krauth, 1988). A significance
60
40
20
D O O U fl
■ »T O
'A W
0
REP
RPA
Fig. 3. Mean percentage of E M G activity relative to the maximum
E M G activity for dynamic contractions for A PB and APLdeep in
various directions. Bars indicate s . d . Hatched columns, APB; black
columns, deep division o f APL. Abbreviations as in Figure 2.
* Correlation significant (P < 0.05).
van ü n d en a a rd e and R. A. B. Oostendovp
H a n cl
%mux. £MC* wgtMiy
Thum b
%my«. EMOactIvicy
100r
200
150 í
100
p w
’J
j
|S:
■xi.
ir
tvè
50
0
-20
HPA
Fig. 4, M e a n p e r c e n t a g e o f E M G a c t i v i t y relative to t h e m a x i m u m
Fig. 7, M e a n p e r c e n t a g e o f E M G act i vi t y relative to t he m a x i m u m
E M G a c t i v i t y f or i s o m e t r i c c o n t r a c t i o n s f o r A P B a n d A P L s u p in
E M G act i vi t y f or d y n a m i c c o n t r a c t i o n s for A P B a n d A P L d e c p in
v a r i o u s d i r e c t i o n s . B a r s i n d i c a t e s . o. H a t e h e d c o l u r n s , A P B ; b l a c k
various directions.
c o l u m n s , s u p e r f i c i a l d i v i s i o n o f A P L . A b b r e v i a t i o n s as in F i g u r e 2.
columns, deep division o f APL.
* C o rre la tio n significant ( P
ï|! C o r r e l a t i o n .significant ( P ^ 0.05).
0.05).
%íimih. 6MG activity
100
Bans i ndi cat e s.D.
Hatched
columns,
APB;
A b b r e v i a t i o n s as in F i g u r e 6.
Th u m b
r "
H a n cl
%fTutx. E'MCi activity
100
••
» •4 «
80
ao
60
60
40
te d
y&
SY't'yA
S/iri
20
•^4
m
a:b
'■"M-'-A
0
Figure
pA
5.
OP
Mean
BA
percentage
AD
of
EMG
activity
||
nPA
HEP
r el at i ve
to
the
m a x i m u m E M G activity for d y n a m ic contractions for APB and
A P L s u p in v a r i o u s d i r e c t i o n s . B a r s i n d i c a t e s.D. M a t c h e d c o l u m n s ,
Fig. 8. M e a n p e r c e n t a g e o f E M G act i vi t y relative to t he m a x i m u m
A P B ; b l a c k c o l u m n s , s u n e r f i c i a l d i v i s i o n o f A P L . A b b r c v u t i o n s as
in F i g u r e 2. * C o r r e l a t i o n s i g n i f i c a n t {.P ^
E M G ac t i v i t y f or i s o me t r i c c o n t r a c t i o n s for A P B a n d A P L s u p in
v a r i o u s d i r e c t i o n s , Bars i n d i c a t e ,s.t). H a t c h e d c o l u m n s , A P B ; bl ack
c o l u m n s , .superficial di vi s i on o f A P I , . A b b r e v i a t i o n s as in F i g u r e 6.
* C o r r e l a t i o n s i gni f i cant ( P ^ 0.05).
%max. SMGActivity
H a n cl
100
rrr^
.„
-
b "rVX* r. r.
Th e mea n values o f E M G signals, relative to the
80
m a x i m u m E M G for isometric as well as for d y n a m i c
contractions, respectively, for A PB and A P L s u p in
60
various directions are presented in Figures 4, 5, 8
a n d 9.
40
A PB
20
fp
DM
l-Jh
A P B (w ithou t crossing the carpal joint) was activated
HPM
in actions o f the t h u m b as well as o f the hand, in the
Fig, 6. M o a n p e r c e n t a g e o f E M G ac t i vi t y r e l a t i v e to t he m a x i m u m
E M G a c t i v i t y f o r i s o m e t r i c c o n t r a c t i o n s f o r A P B a n d A P L d e c p in
v a r i o u s d i r e c t i o n s . B a r s i n d i c a t e s.D. M a t c h e d c o l u m n s , A P B ; bl ack
c o l u m n s , d e e p d i v i s i o n o f A P L . PS, p a l m a r f l e x i o n / s u p i n a t i o n ; D S ,
in a t i o n ;
dorsiflexion/pronation;
P P,
p a Im a r
f Ie x i o n / p r o n a t.i o n ;
DP,
PM , palm ar flexion/midposition; DM ,
;
R P M = repeated
palmar
flexion/
m i d p o s i t i o n . * C o r r e l a t i o n s i g n i f i c a n t ( P 4 0.05).
:y
I was most, strongly
activated in p a l m a r a bductio n, o p p o s itio n and radial
a bd ucti on, as is shown in Figures 2 a n d 4. In the other
directions, only low E M G values were found. In the
in isometric c o n t r a c t i o n s
o f APB, no distinct difference was found in E M G
r
r * t \ /•»
> * r \ / v f i .'" v
q
Coopération between the A PL and the A P B
513
Hand
a b d u c t i o n of the t h u m b ( b o t h 6 0 % o f the m a x i m u m
.k,i
EMG).
140
120
Relationship between A P B and APLcleep
100
T h e a n a to m ic a l c o n n e c t i o n s between À P B a n d
APLcleep are sh ow n in Figure l. Differences between
80
60
40
20
0
RPM
the m e a n percentages o f E M G signals, relative to Lhe
m a x i m u m E M G fo r isometric as well as for d y n a m i c
c o n t r a c t i o n s for b o t h A P B a n d the A P L d e e p in
various directions for the t h u m b a n d the h a n d are
sh ow n in Figures 2, 3, 6 a n d 7.
Fig. 9. M e a n p e r c e n t a g e o f E M G act i vi t y relative t o the m a x i m u m
E M G a c t i v i t y for d y n a m i c c o n t r a c t i o n s for A P B a n d A P L s u p in
v a r i o u s d i r e c t i o n s . Burs i n d i c a t e s.D. H a t c h e d c o l u m n s , A P B ; Bl ack
c o l u m n s , superfi ci al d i v i s i o n o f A P L , A b b r e v i a t i o n s as in F i g u r e 6.
C o r r e l a t i o n si gni f i cant ( P
0.052),
activity between p a l m a r flexion and dorsiflexion (Fig.
6). in d y n am ic c o n t r a c t i o n s the largest values were
found in the p r o n a t e d position o f the forearm (Fig. 7).
Relationship between A P B and superficial A P L
Differences between the m e a n percentag es o f E M G
signals, relative to the m a x i m u m E M G for isometric
as well as d y n a m i c c o n t r a c t i o n s for b o t h A P B a n d
A P L s u p in various directions for the t h u m b and the
h a n d are shown in F i g u r e s 4, 5, 8 a n d 9.
D ISCU SSIO N
Deep division o f A P L
T h e A P L d e e p was most strongly activated in do rsi­
flexion o f the hand, but (without crossing C M C ! ) also
in actions o f the th u m b , especially in radial abduction
(Fii>. 2). In the tested directions o f the thumb the
A P L d e e p , without an insertion into the thumb, was
activated in opposition, radial abducti o n an d r e p o ­
Th e main result o f this s t u d y is a direct functional
relationship between the A P L d e e p a n d APB. T h e
A P L d e e p as well as APB' were a c ti vate d in actions o f
a joint which is n o t crossed by these muscles. T h e
A P L d e e p was a c tivate d in m o v e m e n t s o f the t h u m b
a n d A P B in m o v e m e n t s o f the h a n d , as is s h o w n in
F
.
•
ugure
I
L
sition as is shown in Figures 2 and 4. in the tested
directions of the hand the strongest E M G activity of
the A P L d e e p was found in dorsiflexion of the hand,
regardless of the position o f the forearm, in isometric
APB a n d the A P L d e e p are closely c o n n e c t e d
anatomically, T h e r e f o r e c o o p e r a t i o n between these
muscles in m o v e m e n ts o f the t h u m b or the hand is
very likely. APB is i n n e r v a t e d by the m e d i a n nerve
as well as in d y n a m i c c o n tr a c tio n s (F igs 6, 7). E M G
a n d A P L by the radial nerve, in general it is a s s u m e d
t h a t a separate i n n e r v a t i o n o f muscle parts corres p o n d s to a different function, in this case, to the
contrary, in some c o n d i t i o n s there is a n in terac tion
between 2 muscles i n n e r v a t e d by 2 different nerves.
activity in isometric p a l m a r flexion o f the hand was
negligible, a b o u t 3 -4 % of the m a x im u m E M G (F ig.
6 ).
Superficial division o f the A P L
T h e E M G activity o f the A P L s u p showed no distinct
Relationship between A P B and A P L d e e p
Th e A P L d e e p as well as A P B h ave a n insertion into
difference between actions o f the t h u m b or the hand in
isometric or d y n a m i c contractions. In the tested
the trapezium. A c o o p e r a t i v e f u n c t i o n between these
d irection s o f the thumb in isom etric as well as in
m uscles to stabilise the tra p eziu m to the carp u s is
d yn am ic con tractio n s, this m uscle was m ost strongly
necessary during m o v e m e n ts o f the th u m b or the
activated
hand. B o t h m uscles can be c o n sid e r e d to act as a force
a ctiv a tio n
in
radial
a b d u ctio n
was m u ch
greater
(F igs
than
4,
5).
T h is
for the oth er
directions o f the thumb* In isometric contractions of
the hand no distinct difference in E M G activity was
found between the tested directions. The largest value
was found in d y n a m i c stipulated p a l m a r flexion (Fig.
9), which is similar to the activity in dynamic radial
c o u p le on the trapezium (F ig . 1).
In the tested directions o f the thumb the level o f
E M G activity in p a l m a r a b d u c t i o n was significantly
larger in APB t h a n in the A P L d e e p for isometric as
well as for dyn am ic c o n tr a c tio n s (F ig s 2, 3). A P B can
be considered as a prim e m o v e r for palm ar a b d u ctio n
514
E , van Oudenaarde and R . A. B . Oostendorp
and opposition o f the thumb (Figs 2, 3). The APLdeep
was activated in general to prevent undesired movements o f the hand at the carpal joint. In palmar
abduction o f the thumb the APLdeep has to prevent
a palmar flexion o f the hand, in opposition a
pronation o f the forearm, in radial abduction a radial
deviation o f the hand and in reposition to stabilise the
trapezium to the carpus.
In the tested directions o f the hand in isometric
palmar flexion the EM G activity o f the APLdeep can
be neglected (Figs 6, 7). From this it is clear that APB
showed significantly more EM G activity in palmar
flexion than the APLdeep. The EM G activity from
A PB, which does not cross the carpal joint, can be
explained by its direct connection to the APLdeep,
The insertion from the APLdeep into APB suggests
that both muscles together can be considered as one
long muscle with 2 bellies. In palmar flexion o f the
hand the tendon o f the APLdeep inserting into the
trapezium will lengthen; at the same time the tendon
inserting into APB will relax (Fig. I). It can be
hypothesised that both muscles regulate the tension o f
each other.
In dorsiflexion o f the hand, the amount o f ac­
tivation o f APB is the same as for palmar flexion; the
APLdeep is more strongly activated for dorsiflexion
than for palmar flexion. The APLdeep will now
shorten, and the tendon which inserts into APB will
also regulate the tension o f APB.
Relationship between A P B and the APLsup
The APLsup has, in contrast to the deep division, no
direct anatomical connection with APB. Both muscles
insert into the thumb, the APLsup inserts into MCI
and APB into the proximal phalanx. Both muscles can
thus be considered as prime movers of the thumb.
In the tested directions o f the thumb , differences in
the E M G signals for these 2 muscles were found. APB
was more strongly activated than the APLsup for
palmar abduction and opposition and less strongly
activated than the APLsup for radial abduction and
reposition in isometric as well as in dynamic con­
tractions. The differences between the 2 muscles were
not significant in all directions, but in m ost o f the
individual subjects these differences were present. The
differences in activation between these muscles can be
explained anatomically. APB is situated on the palmar
side o f the hand and is therefore in a better position
for palmar abduction and opposition than APL. APB
can act directly on the CM CI joint. The APLsup is
limited in its function at the same joint because it
passes through the retinaculum and inserts directly
after the joint cleft into MCI. The greater amount o f
EM G activity in radial abduction and in reposition o f
the APLsup can be explained from the position o f this
muscle, which originates mainly from the dorsal
aspect o f the interosseous membrane,
The differences in isometric EMG activity between
both muscles in the tested directions o f the hand are
very small (fewer than 1 %) and not significant (Fig.
8). Both muscles are short and insert directly into the
bones o f the thumb.
The results o f this study may have implications in
clinical assessment and in pathological and surgical
conditions in which these muscles are implicated. The
following come to mind: voluntary muscle testing,
paralysis o f APL in radial palsy, use o f APL in ulnar
and/or median nerve paralysis, and involvement o f
APL, APB and trapezium in conditions such as
tendosynovitis and oesteoarthritis.
Conclusions
From the EM G analysis of APB and APL it appears
that both muscles are activated in the tested directions
o f the thumb as well as in those o f the hand.
Cooperation between these muscles is necessary to
stabilise the trapezium to the carpus in movements of
the thumb. APB is activated in movements o f the
hand to maintain the tension in the deep division o f the
APL. The APLdeep is activated in movements o f the
thumb to prevent undesired movements o f the hand
and forearm. APB and the superficial division o f APL
are the prime movers o f the thumb.
ACKNOWLEDGEMENTS
The authors are grateful to Professor C. C A. M.
Gielen, Department o f Medical Physics and Biophysics, University o f Nijmegen, the Netherlands, for
encouraging this project, supplying some o f the
material and providing hospitality in his laboratory.
The authors wish to thank Peter Snoeren for pro­
ducing Figure 1.
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